The control of energy metabolism in man involves a complex network of neural and humoral signals which include the autonomic nervous system. It is clear that the sympathetic division of the autonomic nervous system has an important role to play in these controls. Measures of epinephrine and norepinephrine are the usual techniques for evaluating sympathetic activity. The role of the parasympathetic nervous system has been much more difficult to evaluate. We have devised a technique of spectral analysis of heart rate variability and drug manipulation of heart rate that permits measures of autonomic input into the heart which we believe reflect autonomic activity elsewhere in the body. We have studied sympathetic and parasympathetic alteration as patients vary in body weight. We now wish to examine the effects on autonomic activity of phentermine hydrochloride, a drug commonly used to promote weight loss.